Functionalization methods coordination sites

Method (i) is a route commonly utilized in monometal nitrosyl complexes. The nitrosyl ligand may function as (formally) a three-electrop donor (NO+) with a linear bonding mode, or as (formally) a one-electron donor (NO ) with a bent (—120°) M-N-0 arrangement. Conversion of the M-NO system to a M-NO system has two effects. First, it increases the metal oxidation state by two second, it generates a vacant coordination site. The dinitrosyl cluster Os3(CO)8(NO)2, which has... 

In method (ii), the group X (e.g., a halogen) may function as a three-electron donor when present as a bridging group, but as a one-electron donor when combined to one metal center. The simple act of bridgeopening creates a vacant coordination site on one metal atom. Behavior of this sort is invoked to explain the stereospecific incorporation of I3CO in the complex Os3(CO)10Cl2. 

While Fe(SCys)4, [2Fe-2S], [3Fe-4S] and [4Fe-4S] clusters all function as one-electron donors or acceptors, the more complex double-cubane [8Fe-7S] cluster that is found only in nitrogenases (see Nitrogenase Catalysis Assembly) has the potential to mediate two-electron transfer processes.Three methods have been employed to functionalize Fe-S centers for substrate binding and activation. The first involves having an accessible Fe coordination site as in the mononuclear Fe centers of nitrile hydratase and SOR, and the [4Fe-4S] clusters at the active sites of hydratases/dehydratases and radical-5-adenosylmethionine (SAM) enzymes.Indeed the recent recognition of the importance of the superfamily of radical-SAM enzymes in initiating radical reactions, via cluster-mediated reductive cleavage of SAM to yield a... 

Pomelli et al. (156) reported a theoretical study using density functional methods to investigate how coordination of a CO2 molecule can assist in the release of formic acid from the catalyst complex in the last step of the catalytic cycle for the hydrogenation of CO2 with rhodium complexes. They find that the presence of a CO2 molecule in their active site model thermodynamically favors the formic acid dissociation from the complex and enhances the reaction rate. This may provide some additional explanation of the dramatic rate increases observed by Noyori and coworkers. 

This indicates that post-synthesis modification of MOFs by coordinating hydrophobic ligands to unsaturated metal sites may be a powerful method to generate new sorbents for gas separation under humid conditions. Amine functionalization to target the water stability of MOFs will be further discussed in the next section. 

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